The purpose of this study is 1) Recruit, screen, assess, and enroll 12 experienced hallucinogen users, who are otherwise currently medically and psychiatrically healthy; 2) Develop sensitive and specific assays for psilocybin and psilocin, the dephosphorylated form of psilocybin; and 3) Generate dose-response neuroendocrine, autonomic, cardiovascular and psychological data for psilocybin in a double-blind, randomized, placebo-controlled fashion.